The present invention relates to a method of manufacturing pellets of hot-melt ink.
Certain types of inkjet printers employ a so-called hot-melt ink i.e. a wax-like ink material that is solid at room temperature and has a melting point, for example, in the order of 100 or 120xc2x0 C. In the printhead of the printer, the ink is heated above it""s melting point, so that droplets of liquid ink can be expelled through the nozzles of the printhead. In order to obtain a high quality of the printed image, the viscosity and hence the temperature of the molten ink in the printhead should be maintained essentially constant. However, since the ink is consumed in the course of the printing process, and the ink reservoir accommodating the liquid ink within the printhead is only of a limited size, it is necessary to supply and melt solid ink while the printer is operating, and the latent heat required for melting the ink tends to decrease the temperature in the ink reservoir. For this reason, it is desirable that the amount of solid ink supplied to the ink reservoir can precisely be controlled and metered, and, to this end, it is advantageous that the ink is supplied in the form of pellets having a predetermined size and shape, e.g. in the form of small spherical pills.
Since the hot-melt ink is a thermoplastic material, the pellets having the desired shape and size can be manufactured by means of a molding process similar to injection molding processes known for manufacturing articles from thermoplastic resins. The molding process should however be adapted to the specific properties of hot melt ink which are in certain respects different from those of thermoplastic synthetic resins. Since the amount of shrinkage, which the hot-melt ink experiences when it is solidified, is comparatively low, and since a certain shrinkage can be tolerated because the final appearance of molded ink pellets is not critical, it is not necessary to apply high locking forces for keeping the mold closed during the molding process. On the other hand, since the hot-melt ink has a relatively low melting point, it tends to solidify immediately when it comes into contact with the walls of the mold cavity. This effect and the fact that the surface of the ink pellet is somewhat tacky, even when the temperature has dropped below the melting point, increases the tendency of the pellet to adhere to the walls of the mold cavity. This makes it more difficult to reliably and reproducingly remove the molded pellet from the molding die.
It is well-known that the removal of a molded product from a die can be facilitated by employing a separating agent which reduces the adherence between the molded product and the walls of the mold cavity. However, a portion of the separating agent inevitably is dispersed or diluted in the molten material, and this is not acceptable in the case of hot-melt ink, because it would cause a deterioration in the quality of the ink. For example, even minute particles of the separating agent, when dispersed in the ink, tends to clog the extremely fine nozzles of the printhead.
It might be considered to use a mold made of a material such as silicon rubber, to which the molten ink does not tend to adhere. However, such materials also become diluted in the molten ink to some extent and therefore the problem of ink contamination is not resolved.
It is an object of the present invention to provide a method for manufacturing pellets of hot-melt ink, in which even when the ink material tends to adhere to the walls of the mold cavity, the pellets can reliably and reproducibly be separated from the mold.
According to the present invention, the ink is shock-cooled with a cooling rate that results in a shrinkage sufficient to cause the solidifying ink to separate from at least one of the mold dies.
Although, as has been stated above, the amount of shrinkage of the hot-melt inks is comparatively low, such inks show a certain shrinkage effect when they are solidified. It has been found that, for the commonly used hot-melt ink compositions, the amount of shrinkage depends on the cooling rate with which the ink is solidified. When cooled more rapidly, the solidifying ink tends to form fewer but larger crystal grains, and this results in a crystal structure in which the solidified ink has a higher density and, hence, experiences a larger shrinkage. It has been found that this shrinkage effect can be utilized for facilitating the removal of the ink pellets from the mold. When the molten ink is poured into the mold cavity, the heat of the ink is dissipated through the walls of the mold, and the solidification process starts at the internal walls of the mold cavity and then proceeds towards the interior of the pellet. Thus, one might expect that the ink would tend to adhere more firmly the walls of the mold cavity when the temperature of the mold is significantly lower than that of the ink. It has been found however that the shrinkage effect mentioned above leads to a relatively uniform contraction of the pellet as a whole rather than to the formation of voids in the interior of the pellet. Thus, the contraction of the pellet solidifying in the mold cavity forces the outer layer of the ink material to withdraw and separate from the wall surface of the mold. As a result, the removal of the pellet from the mold can be facilitated by appropriately controlling the amount of shrinkage, i.e. by appropriately adjusting the cooling rate at which the solidification process takes place. This implies a relatively fast cooling process which is defined as xe2x80x9cshock-coolingxe2x80x9d in the present invention.
In general, increasing the cooling rate will have the effect that the pellets are more readily and more reliably removed from the mold. It should be avoided however to use extremely high cooling rates which would lead to an amorphous solidification of the ink. For certain compositions of hot-melt ink, a too high cooling rate may also lead to high thermal stresses in the ink material, so that the risk of damage to the ink pellets is increased. In addition, a high cooling rate will normally require more powerful cooling equipment and lead to an increased in power consumption. The cooling rate that is optimal in view of these considerations depends on the specific composition of the hot-melt ink and may be determined by experiment. Typical hot melt inks are known inter alia from European patent applications EP 0 856 565, EP 0 978 548, EP 1 067 157 and from unpublished Dutch patent application 1017049.
Shock-cooling in the meaning of the present invention can conveniently be achieved by placing the molds filled with the molten ink in a cooling environment, e.g. a cooling chamber, which is maintained at a suitably low temperature. The temperature in this chamber, which may, for example, be in the range of from +30xc2x0 C. to xe2x88x9230xc2x0 C., for example about xe2x88x9210xc2x0 C., depends on the type of ink, which, in turn, determine the cooling rate.
More preferably, the mold is placed in the cooling chamber before the molten ink is added thereto. Thus, the walls of the mold will already be cool down to approximately the temperature of the cooling chamber before the molten ink is added, and the heat of the molten ink will then be rapidly absorbed by the walls of the mold. In this context, it is also preferable if the mold if made of a material, e.g. a metal, which has a high heat conductivity and a high heat capacity. In order to increase the heat capacity of the mold and hence the cooling rate, a mold having relatively thick walls may be advantageous.
As is generally known in the art, the mold may consist of a lower die and an upper die, with a runner hole being formed in the upper die. In order to remove the pellet from the mold, the mold dies are separated from one another in a vertical direction. If the mold cavity is spherical, for example, it is frequently observed that the pellet tends to stick to the upper die after the mold has been opened. The reason is that a small amount of ink remaining in the runner hole experiences only a small shrinkage effect and therefore adheres to the upper die, so that whole pellet is attached to the upper die. This effect may be eliminated by carefully controlling the amount of ink poured into the mold cavity. On the other hand, this effect may also be used for positively assuring that the pellets stick to the upper die. Then, the pellets may be separated from the upper die by vibrating the die or by injecting it with compressed air or by inserting an ejector pin through the runner hole. The pellets will simply drop out of the upper die and can easily be collected and discharged through a chute or the like.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.